I guess this mod should be part of the regular version - the ability to limit the number of posts which are displayed when answering. This mod for YaBB is called "Ultimate Topic Summaries" and lets you specify the numbers of posts displayed (and if you want them displayed in reverse order which makes a lot of sense to me). Threads with hundreds of posts just become too big to display without this feature. The admin may specifiy the number of posts to be displayed (usually around 10-20) while the user has the option to see the reduced number (default) or all (at the click of a button).
This is already in and has been since YSE 1.3 or something :)